FIG. 1 illustrates an aircraft engine 3 of the unducted fan type, in which the invention can be used. Region 6 of the engine is shown in cross-sectional schematic form in FIG. 2, wherein contra-rotating turbines 9 (decorated with wide hatching) and 12 (narrow hatching) are driven by a hot gas stream 15 provided by a gas generator (not shown). The turbines 9 and 12, in turn, drive contra-rotating fan blades 18 and 21 shown in FIGS. 1 and 2. (The term "contra-rotating" means that turbines 9 and 12, as well as blades 18 and 21 to which they are attached, rotate in opposite directions, as shown by arrows 24 and 27 in FIG. 1.) A view of sub-region 6A in FIG. 2 is shown in perspective form in FIG. 4. In FIG. 4, annulus 29 represents turbine blades 30 in FIG. 2.
The fan blades 18 in FIG. 2 are supported by a polygonal ring 22 in FIG. 4. One type of polygonal ring is described in the U.S. Pat. No. 4,863,352 entitled "Blade Carrying Means", filed by Hauser, Strock, Morris and Wakeman on Nov. 2, 1984. This patent is hereby incorporated by reference.
A cross section 23A of the ring is shown in FIG. 2. The ring 22 is connected to the turbine casing 9A in FIGS. 2 and 4 by schematic brackets 24A in FIG. 2. The ring 22 supports a rotating cowling 28, also shown in FIG. 1, by schematic brackets 25.
The polygonal ring 22 in FIG. 4 includes two types of sections: one type is a blade support section 22B, also shown in FIG. 3, which includes bearings 22D which facilitate pitch change, indicated by arrow 39, of the fan blades 18. The other type of section is a connector section 22A in FIG. 4, including a pair of slender beams 23, which connects neighboring blade support sections 22B.
The fan blades 18 are fastened to the polygonal ring 22 rather than directly to the casing 9A for three principal reasons. One, it is doubtful that a turbine casing 9A of customary design could withstand the centrifugal force applied by the fan blades 18 during operation. Two, different design considerations govern the size and shape of the fan system 33 in FIG. 2 as compared with the turbine system 34. Consequently, it is not expected that the turbine casing 9A would be of a proper shape and location for mounting of the fan blades 18. Three, the casing 9A experiences wide temperature excursions, and it is preferable to avoid mounting the fan blades to a structure of widely variable temperature.
In addition, the engine 3 shown in FIG. 1 can be in the thrust class of 30,000 pounds, which causes a high loading in the fan blades 18. For example, assuming that a total of sixteen fan blades are used on the engine (eight forward blades 18 and eight aft blades 21), then, as a rough approximation, the thrust force of 30,000 pounds, indicated by arrow 35, is shared equally by these sixteen blades: each blade accounts for about 1875 pounds of thrust. If it is assumed that each blade in FIG. 4 is four feet long (dimension 37), and if it is further assumed that the thrust load is concentrated at the midpoint 40 of each blade, then a moment of 1875.times.2, or 3750 foot-pounds must be reacted by each mounting apparatus shown in FIG. 3. Further, this moment is not static, but changes as pitch changes, which is indicated by curved arrow 39 in FIG. 3.